1. Field of the Invention
This invention relates to improvement in earth boring tools, especially steel tooth bits that use hardfacing containing carbide particles to enhance wear resistance.
2. Background Information
The earliest rolling cutter, earth boring bits had teeth machined integrally from steel, conically shaped, earth disintegrating cutters. These bits, commonly known as "steel-tooth" or "mill-tooth" bits, are typically used for penetrating the relatively soft geological formations of the earth. The strength and fracture-toughness of the steel teeth permits the effective use of relatively long teeth, which enables the aggressive gouging and scraping action that is advantageous for rapid penetration of soft formations with low compressive strengths.
However, it is rare that geological formations consist entirely of soft material with low compressive strength. Often, there are streaks of hard, abrasive materials that a steel tooth bit should penetrate economically without damage to the bit.
Although steel teeth possess good strength, abrasion resistance is inadequate to permit continued rapid penetration of hard or abrasive streaks. Consequently, it has been common in the art since at least the early 1930's to provide a layer of wear resistant metallurgical material called "hardfacing" over those portions of the teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast or macrocrystalline tungsten carbide dispersed in a steel matrix. Such hardfacing materials are applied by welding a metallic matrix to the surface to be hardfaced and applying hard particles, usually tungsten carbide, to the molten matrix to form a homogeneous dispersion of hard particles in the matrix. The particles are much harder than the matrix but more brittle. The mixture of hard particles with a softer but tougher steel matrix is a synergistic combination that produces a good hardfacing.
There have been a variety of differing hardfacing materials and patterns, including special tooth configurations, to improve wear resistance or provide self sharpening. An example of a special groove to achieve a deeper deposit and a dual application of hardfacing, one on the crest and another in the groove, is shown in U.S. Pat. No. 2,660,405. A recent invention to increase the durability of teeth with a geometric alteration to the corners of the crests is disclosed in patent application Ser. No. 08/076,716, "Earth Boring Bit Having an Improved Hard-Faced Tooth Structure", filed Jun. 14, 1993, now U.S. Pat. No. 5,351,771. Dual layers of hardfacings, including an underlying layer of cast tungsten carbide applied with the "sweat-on" method and an upper layer applied with the oxyacetylene or atomic hydrogen torch method is shown in U.S. Pat. No. 3,260,579. An example of the use of dual particle sizes of tungsten carbide in a hardfacing to achieve self sharpening is disclosed in U.S. Pat. No. 4,726,432.
There are in general three basic types of tungsten carbide: (1) cast, (2) macrocrystalline and (3) cemented or sintered. All three have been used on earth boring tools, including the gage surfaces of rolling cutter drill bits.
Tungsten is capable of forming two carbides, WC and W.sub.2 C, and cast carbide is typically a mixture of these compounds. The cast carbides are frozen from the molten state and comminuted to selected particle sizes.
Macrocrystalline tungsten carbide is essentially stoichiometric WC usually in the form of single crystals. It is generally tougher and more stable than the cast tungsten carbides, which tend to be brittle and fracture more easily, especially as the supporting steel matrix abrades or erodes.
Sintered or cemented tungsten carbide consists of small particles of tungsten carbide, usually in the range of 1 to 15 microns bonded in a "binder" selected from the iron group metal of cobalt, nickel and iron, commonly cobalt. Tungsten carbide particles and a powder of cobalt are mixed, pressed and sintered near the melting temperature of the cobalt. The resulting mass is comminuted to form particles used for hardfacing.
Since the quest for improved wear resistance of drill bits has been essentially uninterrupted since the 1930's, there are numerous metallurgical mixtures or compositions that coordinate the type of carbide, particle size and matrix composition. The gage surface of a rolling cutter bit constantly rubs against the wall of the borehole and is often exposed to extremely abrasive wear. Consequently, the search for improvements in hardfacing often focuses on the gage surface. A mixture of sintered and single crystal monotungsten carbide particles in an alloy steel matrix is disclosed in U.S. Pat. No. 4,836,307 as one improved hardfacing for gage surfaces. A dispersion of more or less homogeneous sintered tungsten carbide particles in an alloy steel matrix is disclosed in U.S. Pat. No. 3,800,891. Nor has the search been limited to compositions and mixtures of tungsten, because it is known that other carbides such as titanium, are suitable in hardfacing compositions.
One of the most detrimental wear characteristics of steel tooth earth boring bits is the "breakdown" of the heel row teeth at the end of the crest near gage. This "breakdown" causes the teeth to round from the originally manufactured sharp condition, which lowers the rate of penetration (ROP) of a drill bit, requiring more work by the other teeth and shortening the life of the bearings that support the rotatable cutters. There has long been a need for significant improvements in maintaining the tooth geometry of the heel row teeth at the corners of the teeth near gage. This is the primary problem that is the focus of the following invention even though it has greater utility.